1. Field of the Invention
This invention relates generally to recharging systems for implantable medical devices, and more particularly to an external recharging system with forced convection cooling for a battery powered implantable medical device.
2. Description of the Related Art
Many types of implantable medical devices rely on an internal battery pack for primary or backup electrical power. Ventricular assist devices, implantable infusion pumps, pacemakers, and defibrillators represent just a few examples of such devices. Early implantable devices used disposable storage cells almost exclusively, although rechargeable storage cells have also been used in some devices for several years.
Early pacemakers were powered by primary zinc-mercuric oxide cells. Although this system was used for about 15 years, high self-discharge and hydrogen gas evolution presented problems. Furthermore, since these early cells operated at a voltage of 1.5 V, several cells had to be connected in series to obtain the required voltage for pacing.
Over the years, designers have considered many alternative means of power generation and power storage, including primary chemical batteries of all sorts, nuclear batteries, and rechargeable batteries. Consideration was also given to separating the cardiac stimulator system into two parts, a power pack located outside the body that transmits pulses of energy to a passive implanted receiver and a lead. Cardiac pacemakers based on rechargeable nickel-cadmium systems (1.2 V per cell) and rechargeable zinc-mercuric oxide systems (1.5 V per cell) were developed. Although commonly incorporated into many cardiac pacemakers, these systems were unpopular among patients and physicians primarily because the frequency of the recharges was too high (weekly), and the nickel-cadmium system suffered from memory effects which reduced the battery capacity exponentially after each recharge. In addition, the specific energy density of both types of rechargeable batteries was poor, the cell voltages were low, there was no clear state-of-charge indication, and hydrogen gas liberated during overcharge was not properly scavenged either through a recombination reaction or hydrogen getters.
Many present day cardiac pacemakers use non-rechargeable batteries based on a lithium-iodine chemistry. These cells are low current drain devices with a very high energy density and thus can provide a substantial amount of electrical power from a relatively compact sized cell. In addition, lithium-iodine cells are generally not plagued by hydrogen gas evolution. Thus, the housing or can used to enclose the particular implantable device may be hermetically sealed.
Other cardiac stimulators, such as an implantable cardioverter/defibrillator, require higher output currents than lithium-iodine batteries can supply. Such devices typically use a different type of lithium battery employing lithium-silver vanadium oxide chemistry that still functions as a primary battery. Depending upon the severity of the patient's arrhythmia, these implantable defibrillator batteries may last from eighteen months to up to seven years.
Various schemes, such as the use of larger cells and/or the exclusion of collateral or otherwise optional circuitry have been used over the years to lengthen the life of non-rechargeable cells and thus temporarily delay the attendant risks, discomforts, and cost of surgical excision. Larger cell sizes generally yield longer cell life, but also increase the size of the can enclosing the implantable device. Reducing the power consumption of the circuitry in the implantable devices may yield a longer life span for the cell, but will also typically require elimination of collateral circuitry and/or other structure in the implantable device that may provide useful, though not necessarily medically critical functions.
Despite the size and reliability advantages associated with nonrechargeable batteries, there remain several disadvantages associated with these devices. A non-rechargeable cell will, by definition, become depleted within a finite period of time following implantation. Replacement of a depleted non-rechargeable cell requires surgical excision of the entire implantable device.
It is anticipated that future implantable defibrillators will incorporate features such as longer waveform storage, dual chamber pacing, extra sensors, digital signal processing, a combination of defibrillation and drug infusion, among others, all of which will consume extra energy that will reduce the longevity of the storage cell even further. A rechargeable battery that stores adequate energy before recharge would be ideal in such circumstances. Present day advanced rechargeable lithium batteries do not suffer from the same problems as nickel-cadmium or zinc-mercuric oxide batteries. Today's lithium rechargeables have higher voltage and current drain capabilities, and higher capacities with no memory effects. Experiment has shown that these newer batteries require recharge only every 6-12 months, which often coincides with a patient's schedule for routine follow-up medical appointments.
Regardless of the particular chemistry utilized for the rechargeable cell, a recharging system is required to recharge the battery. One such system involves transcutaneous energy transmission. Generally, in a transcutaneous energy transmission system, the implantable medical device is provided with a charging circuit to which energy is transferred by electromagnetic induction. An appliance is placed on or over the skin proximate the implanted device. The appliance is provided with a primary coil. An alternating current in the primary coil induces an alternating current in the charging circuit within the implantable device. The induced alternating current is typically rectified and regulated to provide a direct current for charging the rechargeable cell.
As with nearly all magnetic induction systems, transcutaneous energy transmission gives rise to eddy currents in the housing and various metallic components of the implantable device. The alternating magnetic flux generated by the primary coil not only induces a charging current in the charging circuit of the implantable device, but also induces eddy currents in the device can and various metallic components. The magnitude of the induced eddy currents is a function of the frequency and magnitude of the magnetic flux. An undesirable byproduct of the creation of eddy currents in implantable devices is a temperature increase in the components in which the eddy currents are flowing. The magnitude of the temperature increase in the implantable device is a function of the magnitudes of the eddy currents and the resistances of the components carrying the eddy currents, as well as the total energy transferred during the recharging operation.
Most implantable devices are surrounded by adipose, vascular, and muscular tissues. While it is desirable for heat built up in the implantable device to conduct away through these tissues, implantable device temperatures exceeding certain limits may injure or permanently damage those tissues. The ability of human tissue to withstand hyperthermic conditions is governed by a complex set of factors including the type of tissue involved, the temperature, and the duration of exposure. Although there is no clear cut clinical consensus on the maximum temperature that human tissue can withstand on either an acute or chronic basis without damage, there appears to be a correlation between tissue damage and temperatures above 42.degree. C.
The thermal management of early pacemaker designs seldom required specialized design or unusual charging techniques. Those early designs incorporated relatively small storage cells that required low power levels necessary for recharging. Therefore, those conventional pacemaker designs required more frequent charging, (perhaps weekly), and thus only a relatively small amount of total energy transferred for each charging session. More modern, and high energy consumption systems, such as defibrillators, require a higher transfer of total energy for a given charging. This is due to the higher energy storage requirements of defibrillators as well as the design goal of producing storage cells that require less frequent recharging. Some designs may require recharging every six months. However, each charging session may last two hours or more.
Thermal management for the more powerful rechargeable lithium cell systems has become a matter of concern for designers of new implantable medical devices. Various schemes for heat abatement have been tried with mixed results. Cold packs topically applied to the bare skin transfer heat away by conduction. However, patient comfort is compromised since the icy cold packs must be left on the skin for up to two hours. Two other proposed solutions involve attempts to limit the amount of heat generated rather than transferring the heat that is generated. In one, circuitry is incorporated into the implantable device to manage the charging protocol of the storage cell. The circuitry is complex, consumes space within the device's housing and adds cost to the device. The other proposed solution involves fabricating the device can out of a material that is less conductive and prone to eddy current propagation. However, the available class of biocompatible metallic materials is narrow. No one of those materials exhibits a significantly lower conductivity than the others.
The present invention is directed to overcoming or reducing one or more of the foregoing disadvantages.